1. Field of the Invention
The present invention relates to a preamble receiving apparatus, for example, an apparatus for receiving a preamble which is transmitted by use of a random access channel (RACH) from a transmitting apparatus such as a mobile terminal.
2. Description of the Related Art
In International Standardization Meeting (3GPP), LTE (Long Term Evolution) is a next generation mobile communication standard which is presently being developed. In a case where a mobile terminal (terminal) intends to transmit information to a base station by employing an up link according to the LTE standard, the terminal performs a synchronization operation by employing a random access channel (RACH).
In the LTE standard, a communication operation is carried out while 0.5 [ms] is used as a basic unit (subframe). As a consequence, when the communication operation is carried out, the respective terminals adjust timing in accordance with an interval of 0.5 [ms]. After that, the communication operation is performed. Specifically, the terminal performs the synchronization operation with the base station the instant a power supply of the terminal is turned on, or the instant the terminal recovers from an out-of-service zone. The synchronizing method as described above is mainly divided into the following two steps.
[1] A reception timing synchronization of the terminal using such notification information as SCH (Synchronization Channel) and BCH (Broadcast Channel).
[2] A transmission timing synchronization of the terminal using a RACH. As for the reception timing synchronization of the item [1], each of a plurality of terminals located within a cell of the base station grasps the timing of a predetermined subframe interval (0.5 [ms]) determined by the base station based upon information described in both of the SCH and the BCH, which are notified by the base station to the entire cell. It should be noted that distances between the base station and the respective terminals are different for every terminal. As a result, when viewed based upon absolute times, delays corresponding to the distances of the respective terminals are produced in the timings determined by the base station (refer to FIG. 1). In fact, in a case where the base station performs only a transmission operation to the terminal by employing a down link, even when a delay is produced, it is only necessary that the terminal grasps only the subframe interval, and that the synchronization between the terminal and the base station is made at the level as described above.
Next, description is made on the transmission timing synchronization of the item [2]. In a case where the terminal intends to transmit information to the base station through the up link, a restriction on the synchronization is further added. Description is made of a RACH operation which is performed by the terminal before the terminal executes an up link transmission.
In the current LTE standard, it is required that the timings at which transmission signals from the respective terminals reach the base station are made coincident with each other at the base station. Thus, the terminal located far from the base station must transmit the signal at an earlier stage considering the delay corresponding to the distance, whereas the terminal located near the base station must transmit the signal at a later stage. In order to measure delay times at a time of transmission for the respective terminals, the RACH is employed. Specifically, the respective terminals transmit RACH signals to the base station, the base station calculates delay amounts of the respective terminals by employing the RACH signals, and the base station sends back the calculated delay amounts to the respective terminals (refer to FIG. 2).
The respective terminals adjust transmission timings by considering the delay amounts corresponding to where the respective terminals are located. As a result, the base station can receive the transmission signals from all of the terminals at the same timing (refer to FIG. 3). The delay amounts of the respective terminals at this time are influenced by reciprocating times of the signals. This reciprocating time will be referred to as RTT (Round Trip Time).
Next, description is made on an operation through which a RACH signal transmitted from the terminal is processed at the base station. The RACH signal is transmitted in a different format from that of the normal data communication. FIG. 4 is an explanatory diagram for explaining a general format of a RACH subframe.
As a unit of a subframe, 0.5 ms is defined as one unit (TTI). The subframe has a format composed of a section “TDS”, a section “Preamble”, a section “TGP”, and a section “TDS”. The preamble section corresponds to a section for transmitting the RACH signal (preamble) used to calculate the delay time. The section “TDS” located in front of the preamble section corresponds to a margin (guard time) for preventing another signal to overlap with the RACH signal in a case where another signal outputted before the RACH signal is delayed (refer to FIG. 5). The section “TGP” subsequent to the preamble section corresponds to a margin section for compensating the delay generated due to a difference in the distances of the respective terminals.
In a terminal located immediately below the base station, since no delay is produced at all, the RACH signal having reached the base station will have the same structure as that shown FIG. 4. On the other hand, in a terminal located at an edge of the cell, since maximum delay is produced, the RACH signal transmitted from that terminal reaches the base station under a state where the RACH signal is delayed up to the last of the section “TGP”. FIG. 6 indicates a RACH signal received from a terminal located at the farthest position from the base station. A value (length) of this section “TGP” is determined based upon a maximum cell radius supposed.
Further, the section “TDS” is provided after the section “TGP”. This section “TDS” corresponds to a margin which is provided in order to cope with a multipath delay of the RACH signal itself (refer to FIG. 7). That is, even when the multipath delay is caused in the RACH signal transmitted from the terminal located at the farthest position from the base station, the section “TDS” functions as a guard time for properly receiving the RACH signal.
As described above, the RACH signals transmitted from the terminals have the delay times corresponding to the distances between the terminals and the base station and the multipath at a time when the RACH signals are received at the base station.
The base station calculates the delay times of the respective RACH signals by employing the RACH signals (preamble signals: hereinafter, referred to also as “preamble”) received from the respective terminals. Specifically, the base station calculates the delay time by correlating the received RACH signal with a replica signal thereof.
The RACH receiving process operation is carried out as follows. When an image of the RACH signal (preamble: RACH Preamble) described up to now is correctly illustrated, the RACH signal has a waveform of a certain function as shown in FIG. 8. In general, the waveform of the RACH signal is formed by employing a specific function called “CAZAC”. As shown in FIG. 9, the CAZAC waveform (CAZAC sequence) has a characteristic in that continuity is maintained through a start and end of the waveform. In a RACH correlating process operation of the RACH receiving process operation, a process using this characteristic is carried out.
In the RACH correlating process operation, correlation is calculated between the received RACH signal and the known RACH pattern (replica signal) in the base station. FIG. 10 is an explanatory diagram for explaining the RACH correlating process operation. The correlation between the received RACH signal and the replica signal is calculated, and a waveform (which is called “power profile”) after the correlating process operation is created. In the power profile, a peak is detected from a predetermined temporal section (which is called “search section”). A starting point of the search section is an earliest timing at which the peak appears, which corresponds to the preamble. A length of the search section corresponds to a maximum delay time. In a case where the cell radius is maximum, the length of the search section corresponds to a margin section (namely, TGP+TDS in this example) subsequent to the preamble section. A distance between the starting position (starting point) of the search section and the peak position is calculated as the delay time.
Thus, in a case where the received RACH signal has no delay (for instance, in a case where the terminal is located immediately below the base station), a peak appears at a head (starting point) of the search section in the power profile after the correlating process operation. In contrast, in the power profile acquired from a signal which is delayed with a maximum delay time and is received from a place such as a cell edge, a peak appears in the vicinity of an ending position (end point) of the search section.
As shown in FIG. 11, as sections to be compared with each other in order to make a correlation, the preamble section (FIG. 11A) and another section (FIG. 11B) of the replica signal corresponding to the preamble section are basically employed. While the sections are illustrated by time axes in FIGS. 11A and 11B, there is also a method in which the correlation is made after the section is transformed into a frequency domain by way of discrete Fourier transform (DFT).
It should be noted that when sections are correlated with each other in a time domain, a convolution integration process operation is performed so that a process amount is increased. In contrast, when the time domain is converted into a frequency domain, the convolution integration process operation executed in the time domain can be replaced by a multiplying process operation. As a result, the process amount is decreased. The process amount of “DFT+multiplication+IDFT (inverse discrete Fourier transform)” becomes smaller as compared with the process amount of the convolution in the time domain, so the process operation in the frequency domain is employed in many cases.
The correlating process operations shown in FIGS. 10 and 11 have been carried out based upon an assumption that the received RACH signals have no delay at all. When the received RACH signal has a delay, a problem occurs. FIG. 12 is an explanatory diagram for explaining a process operation when a delay is caused. As shown in FIG. 12A, normally, the RACH signal which is received by the base station has a delay corresponding to the distance from the terminal. As a result, a waveform of the RACH signal is shifted to a rear side of the preamble section. When the preamble section is cut out in this state, then the cut signal is brought into a state where a front half portion thereof is chipped. The cut signal as described above cannot be directly used in the correlating process operation.
In order to solve the problem as described above without changing the cutting section from the received RACH signal, a process operation shown in FIG. 12B is carried out. Specifically, a process operation (hereinafter, referred to as “overlap-and-add (OAA)”) in which a signal sticking out to the margin section (TGP+TDS) provided for compensating the delays is added to the preamble section to be used in the correlating process operation from the front side thereof is carried out.
Conventionally, in the OAA, a reception content in the margin section (namely, TGP+TDS in this example) is cut out. The reception content of this cut section is overlapped with the preamble section under a state where the starting point of the reception content is made coincident with the starting point (target reception timing) of the preamble section. As a result, since the RACH signal has the CAZAC waveform, a rear portion of the added portion of the RACH signal by the OAA of the search section is continuously coupled to the head of the RACH signal received in the preamble section. The pre-process operation as described above is carried out before the correlating process operation. As a result, a continuous RACH signal of a single unit can be derived (cut out) from the cut section (preamble section).
As described above, since the OAA is carried out before the correlating process operation, the delayed RACH signals can be correlated with each other. FIG. 13 is an explanatory diagram for explaining a correlating process operation related to a delayed signal whose pre-process operation has been accomplished. A preamble section is cut out from the delayed RACH signal whose pre-process operation has been accomplished, and the cut delayed RACH signal is compared with a replica signal corresponding thereto so as to acquire a power profile. In the power profile, a portion at which the cut signal waveform coincides with the replica signal waveform appears as a peak. A difference between a position of this peak (namely, preamble reception timing) and a starting point (namely, target reception timing) of a search section is calculated as a delay time (refer to FIG. 14). The base station notifies this calculated delay time value (delay amount) to the terminal. As a consequence, the terminal can grasp the delay time, and thus, can determine the transmission timing to the base station.
Here, a description is made on both the RACH signal transmitted by the terminal and the known replica signal in the base station. In general, a CAZAC series is used as a series for generating the RACH signal. In order to form the RACH signal, several pieces of CAZAC sequences are selected to be used. At this time, in order to separate the respective CAZAC patterns from each other, the respective CAZAC patterns must be perpendicular with one another. When a series length of the CAZAC sequence becomes long, orthogonal patterns with respect to this sequence pattern is increased (refer to FIG. 15).
FIG. 16 shows an example in which 16 pieces of CAZAC patterns are used. Basically, the terminal selects one of the plurality of CAZAC patterns at random, and transmits this selected CAZAC pattern as a RACH signal (preamble). In FIG. 16, a pattern 6 is selected and transmitted.
The base station is notified of 16 patterns of the CAZAC series used in the RACH (there is another case where CAZAC series patterns are notified to the terminal through notification channel such as BCH). In the correlating process operation, replica signals of these 16 patterns are used.
FIGS. 17 and 18 are explanatory diagrams for explaining a correlating process operation performed by the base station. The base station is not notified of the CAZAC pattern contained in the received RACH signal. As a consequence, the base station performs the correlating process operation with respect to all of the CAZAC patterns (namely, 16 sorts of the CAZAC patterns in this example). When the correlating process operation is carried out between matched patterns, a peak is produced in a power profile thereof. On the contrary, a peak is not produced between patterns which are not matched with each other. As a consequence, in the example shown in FIGS. 17 and 18, a delay time can be calculated from a power profile which is obtained by performing the correlating process operation with employment of the replica signals of the pattern 6.
[Non-Patent document 1] “E-UTRA Random Access Preamble Design”, Athens, Greece, Mar. 27-31, 2006, TSG-RAN WG1 #44bis, R1-060998
Noise is contained in an actual waveform of a RACH signal received by a base station. FIG. 19A is a diagram for showing an example of the RACH signal containing the noise, and FIG. 19B is a diagram for showing a case where an OAA is performed with respect to the RACH signal containing the noise. For the sake of explanation, FIGS. 19A and 19B are illustrated in such a manner that the noise is not contained in portions other than a signal transmitted to a preamble section by a terminal. However, the noise is also mixed in the RACH signal (preamble).
In the conventional OAA process operation, a process operation in which all of reception contents (namely, all of reception contents of TGP and TDS in FIG. 19B) in the margin sections subsequent to the preamble section are superimposed and added to the preamble section is carried out only once. At this time, the noise contained in the margin sections is also added to the preamble section. As a consequence, the noise is mixed in the RACH signal (preamble) received within the preamble section, thereby causing a problem in that the characteristics of the preamble are deteriorated.
An improvement in the receiving characteristics of the preamble is preferable not only in a case where the OAA process operation as described above is carried out, but also in the case where a reception side of a communication is synchronized with a transmission side thereof in a proper manner.